The origin of oxygen vacancy induced ferromagnetism in undoped TiO2

Using the full-potential linearized augmented plane wave method, we have investigated the oxygen vacancy defect induced ferromagnetism in both rutile and anatase TiO2. It has been found that the oxygen vacancy induces lattice distortion in rutile TiO2, whereas there is no such meaningful change in the anatase structure. Interestingly, the lattice distorted rutile TiO2 shows an oxygen vacancy induced ferromagnetic state with a magnetic moment of 0.22 mu(B) in the Ti atom neighboring the vacancy site, while only 0.06 mu(B) is observed in the Ti atom in anatase TiO2. We attribute the sizable magnetic moment due to the oxygen vacancy in rutile TiO2 to the charge redistribution owing to lattice distortion. Experimentally measured magnetic hysteresis curves for undoped rutile and anatase TiO2 films clearly display ferromagnetic behavior at room temperature. The observed magnetic strength of the rutile sample turns out to be larger than that of the anatase sample, in accordance with the theoretical calculations.